This is a study of the metabolic circadian rhythms in the rat heart. Such rhythms furnish unique opportunities to examine changing metabolic patterns that normally occur within the heart throughout the day and to investigate in vivo operation of the control of enzymatic systems by hormones, neurotransmitters and substrate and/or product feedback modifiers. Rhythms present an opportunity for investigation of the interactions between physical and social environmental factors and cardiac metabolism. The proposed work will extend previous studies on the factors affecting circadian rhythms of myocardial glycogen. The comparative importance of the glycogen phosphorylase b a interconversion of allosteric activation of phosphorylase b by 5' AMP for the glycogenolytic phase of the glycogen rhythm will be determined. The relation of plasma and myocardial lipids to the glycogen rhythm will be studied. A search will be made for a circadian rhythm of protein synthesis in the heart. The circadian cycles of mean blood pressure and heart rate will be measured and related to the myocardial metabolic rhythms. An exploration will be made for a circadian rhythm of cardiac contractility as expressed by dP/dt and left ventricular end diastolic pressure (LVEDP). The influence of altered light schedules, meal feeding, 48 hr. fasting, exercise and alterations in the social environment on the amplitude and phasing of the myocardial glycogen rhythm will be determined. The rats will be housed in environmental rooms with a controlled light cycle and allowed food and water ad libitum. Current standard analytic procedures will be used on the tissues frozen in sutu. Mean blood pressure of unanesthetized rats will be measured through a chronic indwelling catheter attached to a pressure transducer. Heart rate will be obtained by means of telemetry. Tissue samples will be taken at 0000, 0400, 0800, 1200, 1600 and 2000 hours.